1. Technical Field
The present invention is in the technical field of semiconductor ultraviolet light-emitting devices and lamps. More particularly, the present invention comprises semiconductor ultraviolet light-emitting devices which can emit multiple-wavelength and/or broadband spectral output in the ultraviolet spectrum and lamps based on these devices.
2. Background of the Technology
It is well known that ultraviolet (UV) light with wavelength for 210 nm to 400 nm may be used for disinfecting fluid and air. The UV light may inactivate the pathogens such as bacteria (E. coli), viruses (poliovirus, influenza, hepatitis), protozoan cysts (cryptosporidium, Giardia Lamblia), yeasts and molds by breaking the molecular bonds within micro-organismal DNA, producing thymine dimers in their DNA thereby destroying them, rendering them harmless or prohibiting growth and reproduction.
Currently, mercury-vapor lamps are the most common UV light sources and are classified into low-, medium- and high-pressure bulbs. For long, most of the fluid and air purification community have employed low-pressure UV bulb that generates a single wavelength spectral distribution with the peak wavelength of 254 nm. This is partly due to the fact that the costs of ownership of medium- and high-pressure UV bulbs are much higher than the low-pressure UV bulbs. Another reason responsible for the high popularity of low-pressure UV bulb is based on the assumption that the DNA absorption peak wavelength is around 260 nm, which is very close to the spectrum of the low-pressure UV bulb around 254 nm.
In comparison to the single-wavelength spectral distribution of low-pressure mercury bulbs, the medium- and high-pressure bulbs have about a ten-wavelength spectral distribution in the UV spectrum. However, the spectral energies of medium- and high-pressure mercury bulbs do not overlap the DNA absorption peak wavelength as much as low-pressure bulbs do. Therefore the pathogen inactivation efficiencies of the medium-and high-pressure bulbs were widely thought to be lower than the low-pressure UV bulbs.
However, the true mechanism of the interaction between the pathogens and radiating wavelengths might actually be complex: the pathogen inactivation may further depend on the reaction between proteins, amino acid, different structures and UV light with spectral distributions. In some recent studies, the multiple-wavelength spectral distributions from the medium- and high-pressure mercury bulbs have shown advantages over the single-wavelength spectral distribution from the low-pressure mercury bulb, shown as followed:
1. Several studies have found that the medium-pressure bulbs may have better inactivation effects onto E. coli as compared to the low-pressure bulbs, which is attributed to the multiple-wavelength spectral distribution of the medium-pressure bulbs and is supported by the following experimental observation: (1) E. coli can undergo photorepair following exposure to the low-pressure UV bulb, but no repair was detectable following exposure to the medium-pressure bulb with the same exposure dose. (2) The medium-pressure bulb is found to cause genetic damage as well as affect aromatic proteins, whereas the low-pressure bulb can only cause the genetic damage. [Cutler et al., Animal Health Research Reviews, 1011]
2. Some recent studies have shown that the common pathogens S6633 spores and MS2 Coliphage have different spectral sensitivities. In other words, multi-wavelength and/or broadband spectral distributions are desired to inactivate these two pathogens simultaneously and effectively. [Mamane-Gravetz et al, Environ. Sci. Technol., 1005]
Therefore it is necessary to have multiple-wavelength UV light sources for more efficient pathogen inactivation. In addition, the spectrum of the multiple-wavelength UV light source should be tunable upon the species of the pathogens in order to meet the pathogens' spectral sensitivity and achieve the most efficient pathogen inactivation through mechanisms such as aromatic proteins affection, and genetic damage.
Unfortunately, the spectral outputs of low-, medium- or high-pressure mercury-vapor bulbs are not tunable and cannot meet these requirements.
Other applications such as the curing of inks, coatings and adhesives as well as may also be advanced by introducing multiple-wavelength and/or broadband ultraviolet emissions.
Light-emitting diodes and laser diodes are semiconductor light emitters that can generate light upon a sufficient current injection. The light-emitting diode and laser diode are referred to as light-emitting device (“LEDs”). The wavelength of the light emitted by the LEDs typically depends on the property of the material from which the p-n junction is fabricated and the thin layer structure consisting of the active region of the LED.
Generally, an LED may consist of a substrate, an n-type region deposited on the substrate, a p-type region formed on the n-type doped epitaxial layer, or vice-versa. One or more active regions may be formed between the p-type doped epitaxial layer and the n-type doped epitaxial layer. The active region may consist of structures of single quantum wells, multiple-quantum wells, quantum wires, or quantum dots. A p-type electrode may be fabricated on a p-type doped epitaxial layer, and an n-type electrode contact may be formed on an n-type doped epitaxial layer, in order to facilitate the current flowing to the device from the power source. When an electrical current is applied to the LED, the holes and electrons may be injected into the active region from the p-type doped layer and the n-type doped layer, respectively. The radiative recombination of holes and electrons within active layer of the active region lead to photon generations.
So far, the visible LED has been widely applied to white light generation for illumination applications. Recently, the ultraviolet (UV)-LED has drawn considerable attention to the research and manufacturing. The UV-LEDs with emission wavelength of 250 nm to 400 nm have been realized in the laboratory and available to consumers. However, the current UV-LED only emits a single-wavelength spectral output. Hence in order to realize multiple wavelength and/or broadband spectral output, a number of UV-LEDs with different spectral outputs are needed, leading to potential problems of high cost, large size and non-uniform lifetime.
Accordingly, there still exists a need for devices which emit a multiple-wavelength and/or broadband spectral output in the ultraviolet wavelength range from 210 nm to 400 nm.